Systems for measuring the intra-arterial blood pressure of a patient can be subdivided into two main groups--those which invade the arterial wall to access blood pressure and those which use non-invasive techniques. Traditionally, the most accurate blood pressure measurements were achievable only by using invasive methods. One common invasive method involves inserting a fluid filled catheter into the patient's artery.
While invasive methods provide accurate blood pressure measurements, the associated risk of infection and potential for complications, in many cases, outweigh the advantages in using invasive methods. Because of these risks associated with invasive methods, a non-invasive method, known as the Korotkoff method is widely used.
The Korotkoff method is known as an auscultatory method because it uses the characteristic sound made as the blood flows through the artery to mark the points of highest (systolic) and lowest (diastolic) blood pressure. Although the Korotkoff method is non-invasive, it only provides a measurement of the highest pressure point and the lowest pressure point along the continuous pressure wave. While systolic and diastolic pressure are sufficient for accurate diagnosis in many instances, there are many applications in which it is desirable to monitor and utilize the entire characteristic curve of the blood pressure wave. In these applications, the Korotkoff method is simply incapable of providing ample information. In addition to this limitation of the Korotkoff method, it necessitates the temporary occlusion (complete closing) of the artery in which blood pressure is being monitored. While arterial occlusion is not prohibitive in many applications, there are occasions where the patient's blood pressure must be monitored continuously (such as when undergoing surgery) and accordingly, the prohibiting of blood flow, even on a temporary basis, is undesirable.
Because of the above-mentioned risks involved with invasive blood pressure measurement, and the shortcomings of the Korotkoff method, extensive investigation has been conducted in the area of continuous, non-invasive blood pressure monitoring and recording. Some of these non-invasive techniques make use of tonometric principles which take advantage of the fact that as blood pressure flows through the arterial vessel, forces are transmitted through the artery wall and through the surrounding arterial tissue and are accessible for monitoring at the surface of the tissue. Because the tonometric method of measuring blood pressure is non-invasive, it is used without the risks associated with invasive techniques. Furthermore, in addition to being more accurate than the Korotkoff method discussed above, it has the capability of reproducing the entire blood pressure wave form, as opposed to only the limited systolic and diastolic pressure points provided by the Korotkoff method.
Because the accuracy of tonometric measurements depend heavily upon the method and apparatus used to sense tissue forces, several sensors have been specifically developed for this purpose. For example, U.S. Pat. No. 4,423,738 issued to Newgard on Jan. 3, 1984 discloses an electromechanical force sensor which is made up of an array of individual force sensing elements, each of which has at least one dimension smaller than the lumen of the underlying artery wherein blood pressure is to be measured. Also, U.S. Pat. No. 4,802,488 issued to Eckerle on Feb. 7, 1989, discloses an electromechanical transducer that includes an array of transducer elements. The transducer elements extend across an artery with transducer elements at the ends of the array extending beyond opposite edges of the artery. Additionally, U.S. patent application Ser. Nos. 07/500,063 and 07/621,165 both disclose tonometric sensors for use in determining intra-arterial blood pressure. Each of the above four mentioned patents/patent applications disclose transducers having sensing portions that span well beyond the lumen opening of the underlying artery. One main reason it is advantageous to construct a sensor in this manner is because the arteries of interest are relatively small and difficult to locate. By constructing tonometric sensors which employ a relatively long sensing area, the placement of the sensor by a technician, is not as critical as it would be if the sensor was capable of only sensing along a narrow region.
Although by constructing a tonometric sensor with a long sensing portion, the technician's task is simplified, it introduces certain complexities into the methodology used for determining intra-arterial blood pressure. For example, because the sensor face is made relatively long as compared to the lumen of the underlying artery, only a small fraction of the sensing portion of the tissue stress sensor is overlying the artery, and it is only this portion which is sensing useful forces (i.e. forces which are related to intra-arterial blood pressure). The remaining portion of the sensing portion is in contact with tissue which does not overlie the artery of interest, and accordingly, does not transmit forces to the sensing portion which can be used for determining intra-arterial pressure.
Therefore, in view of the above complexities, when employing tonometric sensors of the type discussed above, before the accurate intra-arterial blood pressure can be determined, a method must be employed for determining which portion of the sensor is best positioned over the artery of interest for determining the intra-arterial blood pressure. One such method is disclosed in U.S. Pat. No. 4,269,193 issued to Eckerle on May 26, 1981. The method disclosed in the '193 patent includes selecting the transducer element which has a maximum pulse amplitude output and then looking to its neighbors and choosing the neighbor having a spacially local minimum of at least one of the diastolic and systolic pressures. Other methods are disclosed in U.S. Pat. No. 4,802,488 issued to Eckerle on Feb. 7, 1989. In the '488 patent the following methods are disclosed, a curve-fit method, a two-humps method, a center-of-gravity method, and a "catch-all" method which includes using one of the three aforementioned methods in conjunction with externally supplied user information (such as sex, height, age, etc.). Also, in U.S. Pat. No. 4,893,631 issued to Wenzel, et al. on Jan. 16, 1990, discloses a method for determining which sensor in an array of sensors best tracks the pulse in an underlying artery using a spacially weighted averaging method. This method employs the steps of finding local diastolic pressure minimums, selecting the number of transducers spanning the local minimums, computing the spacially weighted average from elements centered about the local minimums and computing a weighted average therefrom.
Although the above-referenced methods may yield some degree of success, the Applicants of the present invention believe that a method which is superior to those heretofore disclosed methods employs the use of stress energy. For example, it is believed, that the area of the sensor which is best positioned to determine intra-arterial pressure is that portion which receives the greatest contact stress energy from the tissue overlying the artery of interest.
In addition to the above-referenced contact stress energy transfer methodology, a second methodology is disclosed which uses a tissue flexibility distribution method to determine which portion of the stress sensitive element is best suited to measure intra arterial blood pressure. This approach is based on the idea that the tissue immediately over the artery of interest is more flexible than the tissue remote from the artery of interest. By employing a method which determines the flexibility of the tissue at each portion along the stress sensitive element, it can be determined which portion of the stress sensitive element is best suited to use in computing intra-arterial pressure.
Thus, it is an object of this invention to provide a method for determining which portion of a stress sensitive element is best suited to determine intra-arterial blood pressure.
Two methods are disclosed for achieving this object. The first method includes determining which portion along the length of the stress sensitive element receives maximum energy transfer from the tissue overlying the artery of interest. The second method involves determining which portion of the tissue overlying the artery of interest is most flexible.
By determining which portion of the stress sensitive element receives the greatest energy transfer or by determining which portion of the tissue underlying the stress sensitive element is most flexible, this information can be used to determine which portion of the stress sensing element is best suited for determining intra-arterial blood pressure of an underlying artery.